For an ink-jet printer for printing a paper roll, an ink-jet printer is proposed such that both a paper roll and cut paper can be printed. In Japanese published unexamined patent application No. Hei5-147284, an ink-jet printer of the type is disclosed.
In view of miniaturized requirement, the ink-jet printer of this type has a configuration such that a paper roll and cut paper are carried along a common passage and are printed in a common printing position using a single ink-jet head.
On the other hand, in a POS terminal, a printer for printing a paper roll is generally loaded. In such printers, it is known to a printer which can print both a paper roll and cut paper, a narrower paper roll is used for issuing a receipt or the like, which is narrower in width as compared with the cut paper.
It is conceivable that a part of the cut paper platen surface for defining a printing position for the wider cut paper serves as the surface of a paper roll platen for defining a printing position for the narrower paper roll, and the printing operation is carried out such that when the cut paper is printed by an ink-jet head, the cut paper is overlapped above a paper roll arranged along the surface of the platen for the paper roll.
However, if the configuration is adopted, the following problems will be occurred. That is, a part of the cut paper is carried with it overlapped on the paper roll and the surface of the part is printed by an ink-jet head. Therefore, the surface of the part overlapped with the paper roll of the cut paper is closer to the side of an ink-jet head by at least the thickness of the paper roll, compared with the surface of the other part. As a result, if printing for one line of the cut paper is considered, a gap with the platen is different between the part overlapped with the paper roll and the other part. If the gap with the platen is different, an effect due to dispersion in a position in which an ink droplet jetted from an ink-jet head reaches the surface of the cut paper and a jetted direction becomes large and the quality of printing is deteriorated.
Further, as a paper roll is rolled, paper pulled out of a roll curled and is curved in a direction perpendicular to the surface of the paper roll. Therefore, even if a gap between the print head and the paper roll could be maintained in constant, a gap between the print head and the cut paper further varies between an overlapped part and the other part of the paper roll, because a paper roll carried between a print head and a platen is easily curved and is protruded on the side of the print head.
As described above, when a gap varies, not only the quality of printing is deteriorated but also the printed face of paper comes in contact with a head and the printed face may be contaminated by ink and others.
Further, a transmission mechanism includes mainly a gear train which is used for a mechanism for feeding a paper roll, however, when the carriage of the paper roll is stopped, a platen roller coupled to the last section of the gear train is to be slightly rotated freely forward and backward because of a backlash and others of the gear train for transmitting torque from a driving motor to the platen roller. If holding current is turned off to reduce the generation of heat when a driving motor is stopped, a gear train coupled to the driving motor is easily turned rotatable.
In such a state, a platen roller coupled to the last section of the gear train can be rotated and tensile force acting upon a paper roll carried through the platen roller is released. As a result, the paper roll is loosened because the paper roll is curled and is lifted up from the platen roller.
Hereby, the paper roll in the vicinity of the platen roller is greatly curved on the side of a print head and a gap between the print head and cut paper further varies between a part of the cut paper overlapped with the paper roll and the other part of the cut paper.
In a printer for printing on both a paper roll and cut paper, it is desirable to miniaturize the printer that a common driving motor is used for a driving source for both paper carriage mechanisms. A mechanism for transmitting a rotation to different carriage mechanisms using a common driving motor is disclosed in Japanese published unexamined patent application No. Heil-249472 for example.
The lift of a paper roll is also caused in case that switching means is switched to a cut paper carriage mechanism in a printer such that the rotation of a common driving motor is transmitted by switching between a paper roll carriage mechanism and a cut paper carriage mechanism by the switching means.
As a gear train constituted of the paper roll carriage mechanism is freely rotatable by the switching, a platen roller coupled to the last section is freely rotatable.
The present invention is made in view of the above and the object is to provide a printer which can print on both a paper roll and cut paper and can keep a gap with a platen fixed when cut paper is printed in addition.